The production of geothermal energy for generating electricity is limited by the need to obtain sufficient temperatures (currently about 300.degree. F.) and heat flow. While in principle such temperatures can be gotten almost anywhere if one drills deep enough, in practice drilling to these depths becomes prohibitively expensive. The production of electricity from geothermal power becomes the search for geological anomalies which are sufficiently hot and shallow to be economically competitive. When found, such anomalies involve the mining of either fluids or heat.
Fluid Mining
Producing geothermal energy from naturally occurring hot water or steam involves the simultaneous occurrence of several contingencies: hot rock at a sufficiently small distance below the surface, the presence of water or steam, and sufficiently porous rock that the flow of water or steam to the well and the heating of the hot rock can be maintained. While the production of steam is more desirable than that of hot water, it should be recalled that a geyser is a hot spring with an insufficient water supply. Despite recycle and reinjection of any unused water, depletion is inevitable. Such depletion can be estimated by adapting the usual methods used for gas and oil.
Heat Mining
Heat mining methods provide their own working fluids rather than rely upon natural ones.
The approach known as Hot Dry Rock involves finding hot rock at an accessible depth, drilling at least two wells, and fracturing a connection between them by applying hydraulic pressure. Cold water is then passed down one well and hot water emerges from the other after being heated by its passage through the heat transfer area formed by the fractures in the hot rock. The thermal stresses in the hot rock as it cools should open cracks exposing additional passageways for contact with the hot rock. On the other hand, the continued expansion of the passageways by the contracting rock as it cools will channel the liquid to cooler regions. This results in a complex system, the long-term behavior of which is not well understood. The economic effect of such uncertainty must be added to the cost of drilling deep wells into very hard rock.
An alternative to Hot Dry Rock was offered by Jacoby (U.S. Pat. No. 3,676,073) who proposed to dissolve a cavern in salt domes and produce energy by circulating a fluid which would be heated by thermal conduction through the salt to the cavern walls. Problems include the cost of dissolving the cavern and providing the fluid (presumably a hydrocarbon or other organic liquid which would not dissolve the salt which formed the cavern), achieving a useful circulation pattern, and preventing the cavern from closing.
Calculations by Altschuler in an attempt to determine the energy output as a function of cavern size and thus estimate cost showed that the smaller the diameter, the larger the heat flux through the cavern wall per unit of cavern height. This implied the somewhat surprising result that as the cavern diameter decreased, the heat flux increased faster than the cavern wall area decreased. This is explainable by the fact that a unit area on a plane wall will receive the heat flux from other volume elements which pass through areas of equal size. For a unit area on the wall of a circular cylinder, the heat flux from other volume elements will pass through areas of increasing size as the distance from the cavern wall increases. This effect will increase as the cavern diameter decreases, so that a small diameter well becomes a quite efficient means of extracting heat from a geological formation if it can be installed. Once the outer well pipe is in place in the formation, an insulated inner pipe with an open end is installed, thus forming a double pipe heat exchanger. The production of geothermal energy with a double pipe heat exchanger has been described by the Futures Group in a publication for the National Science Foundation (1975). A possible design of the internals of such a system was published by J. H. Warren and R. L. Whitelaw (1975). More recently such a system, dubbed a Downhole Coaxial Heat Exchanger, has been installed and tested in a hot volcanic rock formation in Hawaii albeit entirely by drilling to rather shallow depths (about 3,000 ft.) as described in several publications by Koji Morita et al.
Altschuler proposed (U.S. Pat. No. 4,052,857) to install a double pipe heat exchanger in a salt dome by taking advantage of the plastic behavior of salt by:
1. Drilling and casing a well into a salt dome to a depth at which the salt behaves plastically; PA1 2. Inserting a pipe filled with removable weights and fitted with a closed, pointed end into the well (this outer well pipe which fits inside the casing is filled with water or brine in addition to the weights to help prevent its collapse); PA1 3. Sinking the pipe through the plastic salt to a depth at which the energy output is maximized (ideally, this would be the bottom of the salt dome or formation, i.e., the base salt from which the dome arises); PA1 4. Removing the weights and installing internals (the inner well pipe and insulation) to convert the system into a double pipe heat exchanger.
Because of the requirement that the salt be sufficiently plastic to allow the weighted pipe to sink quickly enough to be economically feasible and because of the uncertainty in estimating a value of the "effective viscosity" of salt as a function of temperature, this method is restricted to salt domes where the salt is believed to be sufficiently hot immediately upon entering the dome. This limits the method's applicability to "deep" domes (where the top of the salt is greater than 10,000 ft. below the surface) or some of the deeper "intermediate" domes (where the top of the salt is 4,000 to 10,000 ft. below the surface). Not only does this limit the area of the resource available, but drilling through the extensive overburden becomes the major cost item of this geothermal system.